This invention relates generally to a sensor system for remote detection and imaging of objects in a backscattering medium such as air or water. More particularly, this invention relates to a method and apparatus for detecting, locating and/or imaging underwater objects such as mines and submarines from an airborne platform using a novel imaging lidar (light detection and ranging) system which employs a method for improving the imaging of targets being viewed in reflection using bistatic operation of the imaging lidar system.
It is desirable in a number of military and civilian applications to search a volume within a backscattering medium for the presence of certain targets. For instance, moored or bottom mines deployed in ocean shipping lanes are a hazard to navigating ships used both for military and for commercial purposes. For other civilian applications such as law enforcement on the ocean, it is desirable to detect the presence of submerged fishing nets or drug-carrying containers used in smuggling contraband. In or near harbors and beaches, it is also desirable to detect submerged obstructions, cables, pipelines, barrels, oil drums, etc. In strictly military applications, anti-submarine warfare demands an effective means of detecting and locating submarines.
Presently, cumbersome and time consuming wire line devices must be used for detecting underwater targets from remote airborne locations. These devices are lowered into the water and of course, are easily subject to damage and loss. Also, wire line devices make target searching relatively slow and can only detect targets without providing visual imaging. An important and novel system for remote detection and imaging of objects underwater (or objects obscured by other backscattering media which are at least partially transmitting to light such as ice, snow, fog dust and smoke) from an airborne platform has been described in U.S. Pat. No. 4,862,257 and U.S. patent application Ser. No. 256,778 filed Oct. 12, 1988, now U.S. Pat. No. 5,013,917, both of which are assigned to the assignee hereof and incorporated herein by reference. The imaging lidar system of U.S. Pat. No. 4,862,257 utilizes a laser to generate short pulses of light with pulse widths on the order of nanoseconds. The laser light is expanded by optics and projected down toward the surface of the water and to an object or target. U.S. application Ser. No. 256,778 relates to an imaging lidar system intended for night vision.
Imaging lidar systems of the type described hereinabove are also disclosed in commonly assigned U.S. patent application Ser. No. 420,247 filed Oct. 12, 1989 (now U.S. Pat. No. 4,964,721), and U.S. patent application Ser. No. 364,860 filed Jun. 12, 1989 (now U.S. Pat. No. 4,967,270, both of which are incorporated herein by reference. USSN 420,247 relates to an imaging lidar system which controls camera gating based on input from the aircraft onboard altimeter and uses a computer to thereby adjust total time delay so as to automatically track changing platform altitude. USSN 364,860 relates to a lidar system employing a plurality of gated cameras which are individually triggered after preselected time delays to obtain multiple subimages laterally across a target image. These multiple subimages are then put together in a mosaic in a computer to provide a complete image of a target plane preferably using only a single light pulse.
USSN 565,631 filed Aug. 10, 1990 which is also assigned to the assignee hereof and fully incorporated herein by reference, relates to an airborne imaging lidar system which employs multiple pulsed laser transmitters, multiple gated and intensified array camera receivers, an optical scanner for increased field of regard, and a computer for system control, automatic target detection and display generation. USSN 565,631 provides a means for rapidly searching a large volume of the backscattering medium (e.g., water) for specified targets and improves upon prior art devices in performance as a result of having more energy in each laser pulse (due to simultaneous operation of multiple lasers) and a more sensitive detection system using multiple cameras. The several cameras may be utilized to image different range gates on a single laser pulse or several cameras can be gated on at the same time to provide independent pictures which can then be averaged to reduce the noise level and improve sensitivity. Both of these improvements result in higher signal-to-noise ratio and thus higher probability of detection or greater range of depth capability.
In accordance with the imaging lidar systems of the type described above, targets are detected by their contrast with the light scattered or reflected back from the surroundings. If the target falls within a gate, it will be seen as a bright spot if its reflectivity is greater than the surrounding water; and either indistinguishable or as a dark area if its reflectivity is equal to or less than the surrounding water. If a target is above the gate, obscuration occurs, and in this case, since the obscuration represents a limiting case (no photons received) the signal to noise ratio is determined by the intensity of the surrounding light backscattered from the water.
Airborne imaging lidar systems fielded to date have been monostatic. In other words, the system's transmitter (laser) and receiver (camera) optics are colocated and coaxial. In a monostatic lidar system, the light scattered back from the gated area returns along the same path as it started from the transmitter. This 180.degree. backscatter occurs at a peak in amplitude, and thus represents a maximum which occurs in backscattered light. This is the optimum arrangement for objects viewed in obscuration. As described in "Marine Optics", N. G. Jerlov, Elsevier Oceanography Series 14, p. 34, Elsevier, New York (1976), this peak in backscattering is symmetric around 180.degree.. The intensity of this backscattering can decrease an order of magnitude at deflections as small as +/- 10.degree. from 180.degree.. As a result, the monostatic imaging lidar systems of the prior art are not well suited for imaging a target when viewed in reflection.